Use Rails environments instead of DB configurations

In the admin forms for Spree::PaymentMethod and Spree::Tracker the collection used for the environment field is the database configuration keys. This is wrong and it becomes quite apparent in the case of applications that connect to more than one database. This commit replaces said collection with a collection of the available environment configurations in '/config/environments/'.

Closes #5170
add specs for rails_environments helper method introduced in #5170